The overall objective of this proposal is to test the hypothesis that calcium supplementation with calcium citrate would be more effective than calcium carbonate in preventing bone loss and the development of osteoporosis in postmenopausal women, because of its greater calcium bioavailability and alkali load, and its reduced risk for stone formation. This hypothesis will be tested by comparing the effects of long-term treatment (4 yrs) with the two calcium salts (1 and 2 g/day) in 192 postmsenopausal women without fractures (early post-menopause, late and elderly). Aim I will assess pathophysiological consequences of calcium suplementation, in order to show that calcium treatment suppresses parathyroid function, inhibits bone resorption and produces a low turnovr state of bone. An assessment of bone mass in vivo will be obtained from single and dual photon absorptiometry and CT scan. Aim 2 will seek to obtain histomorphometric confirmation from iliac crest biopsy in selected patients. Aim 3 will pursue metabolic calcium balance in selected patients in order to show that improvement in calcium balance is correlated with hormonal-metabolic changes and with histomorphometric picture. Aim 4 will assess potential complications of therapy. In particular, it will carefully perform urinary crystallization studies in order to show that calcium citrate therapy confers less stone-forming risk than calcium carbonate supplementation.